Proceeding Book

Proceeding Book Geochemistry of Iron Ore at Wadi As Shati, SW Libya: Implications on Origin, Depositional Environment, Paleooxygenation, Paleoclimate and Age Osama R. Shaltami1, Patrizia Fiannacca2, Fares F. Fares1, Farag M. EL Oshebi1, George D. Siasia3 and Hwedi Errishi4 1Department 2 Department

of Earth Science, Faculty of Science, Benghazi University, Libya

of Biological, Geological and Environmental Sciences, Faculty of Science, University of Catania, Italy 3International 4Department

Braking and Railway Equipment (IBRE), France

of Geography, Faculty of Arts, Benghazi University, Libya

SGA 2017-14th Biennial Meeting of Society for Geology Applied to Mineral Deposit, Québec, Canada

Abstract

Introduction The oolitic ironstone bed represents the upper parts

Wadi As Shati is situated within the Murzuq Basin of

of both Dabdab and Tarut formations, while it is characteristic

southwest Libya. The Wadi As Shati area is mostly desert. It

of the middle part of Ashkidah Formation. The detected iron

borders on Nalut in northwest, Jabal Al Nafusah in the north, Al

minerals are goethite, siderite, hematite, limonite, magnetite,

Jufrah in the east, Sabha in the southeast, Wadi Al Haya in the

chamosite and pyrite. Generally, oolitic ironstone deposition

south, Ghat in the southwest and Illizi Province of Algeria in the

occurs in shallow water lagoonal environments. Fe2O3

west. The study area is the eastern area of a part of the Wadi

represents between 43.57 to 64% of all ironstones contents.

As Shati, between longitudes 13o 45’ 00” and 14o 15’ 00” E and

The discrimination diagrams point to a hydrogenous source for

latitudes 27o 20’ 00” and 27o 40’ 19’’ N (Fig. 1). The three

Fe mineralization. Climatic conditions of semi-humid to semi-

formations; Dabdab, Tarut and Ashkidah, represent the most

arid prevailed during the deposition of the ironstones. The

important iron ore bearing cycles of sedimentation in the Wadi

molybdenite ages for Dabdab, Tarut and Ashkidah formations

As Shati area. The distribution of the three formations in the

are 379.2 ± 1.1 Ma (Fransnian, Late Devonian), 362.5 ± 1.1

study area is shown in Fig (2). The oolitic ironstone bed

Ma (Famennian, Late Devonian) and 355.6 ± 1.1 Ma

represents the upper parts of both Dabdab and Tarut

(Tournaisian, Early Carboniferous), respectively.

formations, while it is characteristic of the middle part of Ashkidah Formation (Fig. 3). This bed was originally named

Keywords: Geochemistry, Ironstones, Origin, Depositional

the ore bearing member “L” in the Dabdab Formation, ore

Environment, Paleooxygenation, Paleoclimate, Age Dating,

bearing member “A” in the Tarut Formation and ore bearing

Wadi As Shati, Murzuq Basin, Libya.

member “B” in the Ashkidah Formation (Seidl and Rohlich, 1984).

Fig. 1: Location map of the study area

Page 37


Fig. 2: Geological map of the study area showing the distribution of Dabdab, Tarut and Ashkidah formations (modified after Seidl and Rohlich, 1984)

Page 38


Fig. 3: Composite columnar section of Dabdab, Tarut and Ashkidah formations in the study area The present work attempts to characterize the mineral

not enough. The age and origin of these iron ores has been

and chemical compositions of the iron ores at Wadi As Shati,

matter of debate.

SW Libya, with especial emphases on origin, depositional environment, paleooxygenation, paleoclimate and age dating

Methodology

of these sediments. As far as the authors are aware, the

Eighteen representative samples of

published data on the oolitic ironstones in the study area are

the oolitic

ironstones were collected from Dabdab, Tarut and Ashkidah

Page 39


formations (six samples of each formation, see Fig. 3). All the

contain high amount of MnO (3.45%, in average), while the

samples were prepared as polished thin sections. The thin

MnO content is relatively low in the samples of Tarut and

section preparation was done in the Thin Section Lab, Toul,

Ashkidah formations (0.69, in average). Oolitic ironstones are

France. The mineral composition of the studied samples was

recognized as being enriched in many trace elements such as

determined by using petrographic study under transmitted

V, Ba, Sr, Co, Zr, Y, Ni, Zn, and Cu (Tobia et al., 2014). In

polarizing and reflected light microscopes.

addition, anomalous P, V, Cr, Ni, Zn, As, Mo, and U are commonly correlated with Fe-oxyhydroxides (Salama et al.,

The major oxide contents were determined by atomic

2012). The studied samples contain high concentrations of Ni,

absorption spectroscopy. Loss on ignition (LOI) was measured

Co, V, Cr, Mo, Pb, As Zr, Nb, Th, U and REE. The REE are

from the total weight after ignition at 1000°C for 2 h. Trace

normalized to Post-Archean Australian Shale (PAAS, Taylor

element contents were determined by inductively coupled

and McLennan, 1985). PAAS-normalized REE patterns of the

plasma-mass

These

studied samples show distinctive positive Ce- and Eu-

analyses were done in the ACME analytical laboratories of

anomalies and enrichment in the HREE over the LREE (Fig.

Vancouver, Canada. The Re and Os concentrations of three

5).

spectrometry

(ICP-MS)

technique.

molybdenite samples were determined by negative thermal ionization mass spectrometry (N-TIMS) using a Finnigan MAT-

Classification of Oolitic Ironstones

262 at the Institut de Physique du Globe de Paris, France.

Oolitic ironstones are sedimentary rocks with >5% ooids and >15% iron, corresponding to 21.4% Fe2O3 (Petranek

Results and Discussions

and Van Houten, 1997; Mucke and Farshad, 2005). Oolitic

Mineral Composition

ironstones accumulated throughout the Phanerozoic Eon, from

On the basis of mineral composition, the studied

the Cambrian to the Recent (Petranek and Van Houten, 1997),

oolitic ironstone beds can be divided into two facies:

following the earlier Precambrian banded iron formations. More than 500 deposits are known, summarized as Phanerozoic

1) Reduced Facies (Lower Facies): In the Dabdab Formation,

oolitic ironstones. They were especially common in the

this facies contains pyrite, chamosite and siderite (Fig. 4),

Ordovician and the latter part of the Silurian, Devonian and

while the reduced iron minerals in the Tarut and Ashkidah

again in the Jurassic and Cretaceous. In contrast, only a few

formations are magnetite, chamosite (see Fig. 4) and siderite.

occurred in the Cambrian, Permian, Triassic, or in the Upper

The reduced facies in all formations contains small amounts of

Cenozoic (Petranek and Van Houten, 1997).

quartz, chlorite, molybdenite, apatite, graphite, zircon and rutile (see Fig. 4).

In the Mediterranean countries, oolitic ironstone deposits of Paleozoic age are documented in France, Algeria

2) Oxidized Facies (Upper Facies): In the Dabdab Formation,

and Libya (Chauvel and Guerrak, 1989). James (1992) showed

the oxidized facies contains limonite and goethite (see Fig. 4)

that Phanerozoic ironstones can be distinguished from

with lesser amounts of gypsum, quartz, kaolinite and

Precambrian iron formations by using the SiO2–FeO+MgO–

manganese oxides (bixbyite, jacobsite and cryptomelane, see

Fe2O3 diagram (Fig. 6). According to Evans (1993) the

Fig. 4), while in the Tarut and Ashkidah formations, this facies

Phanerozoic ironstones can be classified into two types:

contains hematite and limonite (see Fig. 4) and small amounts of quartz and kaolinite.

1) Clinton Type: This forms massive beds of oolitic hematitechamosite-siderite rock. The Fe2O3 content is about 40-50%

Geochemistry

and they are higher in Al2O3 and P2O5 than banded iron

The concentrations of major oxides and trace

formation (B.I.F.). They also differ from B.I.F. in the absence of

elements of the studied ironstones are given in tables (1-2).

chert bands, the SiO2 being mainly present in iron silicate

Fe2O3, SiO2 and Al2O3 represent between 76.72 to 86.30% of

minerals with small amounts as clastic quartz grains. The

all ironstones contents. The Dabdab Formation samples

Clinton type is common in rocks of Cambrian to Devonian age.

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Fig. 4: Photomicrographs of (a) goethite (sample D5), (b) siderite (sample D1), (c) hematite (sample T4), (d) limonite (sample S6), (e) magnetite (sample T2), (f) chamosite (sample S1), (g) pyrite (sample D3) (h) quartz (sample T4), (i) zircon (sample S3), (j) apatite (sample S1), (k) rutile (sample T2), (l) kaolinite (sample D5), (m) molybdenite (sample T3), (n) graphite (sample S1), (o) gypsum (sample D6), (p) chlorite (sample T3), (q) bixbyite (sample D4) and (r) jacobsite altered to cryptomelane (sample D6)

Table 1: Major oxide concentrations (wt%) of the oolitic ironstones Formation Member Facies Sample No. SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O SO3 Cl P2O5 LOI Total

Dabdab L Reduced

Tarut A Oxidized

Reduced

D1 D2 D3 D4 D5 D6 T1 T2 17.29 17.00 17.45 19.00 19.19 18.88 22.87 22.60 0.61 0.53 0.66 0.69 0.70 0.67 0.78 0.73 7.33 7.12 7.53 9.87 9.94 9.58 14.75 14.53 61.68 62.08 61.11 54.44 54.21 54.70 44.40 45.00 3.83 4.00 3.67 3.08 3.00 3.13 0.46 0.51 0.22 0.25 0.26 0.18 0.20 0.20 1.09 1.11 0.51 0.55 0.57 1.44 1.39 1.37 1.05 0.97 0.10 0.07 0.14 0.19 0.21 0.17 2.17 2.00 0.19 0.14 0.21 0.41 0.44 0.39 1.11 1.05 0.20 0.31 0.18 0.69 0.55 0.71 0.06 0.10 0.13 0.16 0.15 0.08 0.08 0.08 0.92 0.90 0.88 0.93 0.95 1.22 1.13 1.19 0.22 0.28 7.00 6.82 7.07 8.66 8.92 8.88 10.08 10.17 99.97 99.96 99.95 99.95 99.96 99.95 99.96 99.95

Ashkidah B Oxidized

T3 22.75 0.75 14.69 44.72 0.48 1.15 1.11 2.05 1.09 0.09 0.88 0.29 9.91 99.96

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T4 16.89 0.50 7.09 52.84 1.18 0.72 2.55 0.08 0.11 0.96 0.23 1.80 15.00 99.95

T5 16.68 0.48 6.93 53.11 1.21 0.87 2.41 0.08 0.10 0.98 0.24 1.91 14.96 99.96

Reduced T6 16.50 0.46 6.81 53.42 1.25 0.89 2.47 0.08 0.09 0.98 0.22 1.90 14.88 99.95

S1 10.45 0.23 5.24 63.67 0.62 0.28 2.44 0.06 0.07 4.21 0.19 0.77 11.73 99.96

S2 10.91 0.25 5.55 63.22 0.53 0.30 2.39 0.06 0.08 4.06 0.27 0.80 11.53 99.95

Oxidized S3 10.31 0.21 5.13 64.00 0.69 0.33 2.47 0.06 0.07 4.27 0.20 0.83 11.39 99.96

S4 23.21 1.33 10.11 43.57 0.40 0.14 0.10 2.37 1.23 0.14 0.09 1.49 15.78 99.96

S5 23.09 1.25 10.05 43.80 0.44 0.17 0.11 2.29 1.20 0.17 0.10 1.33 15.97 99.97

S6 23.00 1.19 9.98 43.91 0.49 0.20 0.11 2.24 1.18 0.19 0.10 1.37 16.00 99.96


Table 2: Trace element concentrations (ppm) of the oolitic ironstones Dabdab L

Tarut A

Reduced D1 126.28 24.71 864.38 176.08 69.29 23.28 19.96 22.84 35.97 169.97 2.46 115.14 1.12 25.46 36.28 5.32 68.29 26.05 206.11 14.27 79.08 18.73 4.63 18.49 3.29 17.81 4.28 11.78 1.69 11.15 1.79

D2 126.68 25.11 864.78 176.48 62.69 23.68 20.36 23.24 36.37 220.88 2.37 166.45 1.03 26.86 36.68 6.72 68.38 26.14 206.20 14.36 79.17 18.82 4.72 18.58 3.38 17.90 4.37 11.87 1.78 11.24 1.88

Oxidized D3 125.71 24.14 863.81 175.51 61.72 22.71 19.39 22.27 35.40 243.74 1.23 188.91 0.89 26.89 35.71 6.75 68.20 25.96 206.00 14.18 78.99 18.64 4.54 18.40 3.20 17.72 4.19 11.69 1.60 11.06 1.70

D4 194.04 192.47 557.14 295.84 15.11 91.04 12.78 27.66 28.79 121.21 1.70 66.38 0.75 111.72 24.05 11.58 76.55 32.86 211.89 16.43 78.21 18.65 4.59 19.88 3.40 19.00 4.34 12.64 1.75 11.63 1.88

D5 193.81 192.24 556.91 298.61 14.82 90.81 12.49 25.37 28.50 186.88 1.37 132.25 1.03 111.49 23.81 11.35 76.62 32.93 211.96 16.50 78.28 18.72 4.66 19.95 3.47 19.07 4.41 12.71 1.82 11.70 1.95

Ashkidah B

Reduced D6 194.30 192.73 557.40 299.10 15.31 91.30 12.98 29.86 28.99 164.32 0.81 109.49 0.47 111.98 24.30 11.84 76.48 32.79 211.82 16.36 78.14 18.58 4.52 19.81 3.33 18.93 4.27 12.57 1.68 11.56 1.81

T1 114.60 22.13 957.00 162.45 35.00 11.60 2.67 25.55 18.68 80.67 1.16 25.84 0.45 22.88 59.60 2.74 77.47 37.14 226.48 17.40 81.81 18.50 4.74 19.72 3.28 18.95 4.51 13.00 1.82 11.31 1.90

T2 115.20 22.63 957.60 163.05 35.61 12.20 3.28 26.16 19.29 230.78 1.27 175.93 0.93 23.38 60.20 3.24 77.40 37.05 226.41 17.33 81.74 18.43 4.67 19.65 3.21 18.88 4.44 12.93 1.75 11.24 1.83

Oxidized T3 114.92 22.35 957.32 162.77 35.33 11.92 3.00 25.88 19.00 189.54 2.03 134.76 1.29 25.10 59.92 4.96 77.54 37.21 226.55 17.47 81.88 18.57 4.81 19.79 3.35 19.02 4.58 13.07 1.89 11.38 1.97

T4 198.04 196.47 565.44 295.89 13.45 95.04 11.12 34.00 27.13 185.53 1.14 131.70 0.94 107.72 78.00 7.58 58.15 27.37 192.96 13.92 67.44 15.72 4.09 17.34 2.89 16.29 4.16 10.82 1.64 10.39 1.63

T5 198.31 196.74 565.71 296.16 13.72 95.31 11.39 34.27 27.40 162.97 0.58 108.94 0.38 197.49 78.31 8.35 58.08 27.30 192.89 13.85 67.37 15.65 4.02 17.27 2.82 16.22 4.09 10.75 1.57 10.32 1.56

Reduced T6 198.62 197.05 566.02 291.47 14.00 95.62 11.67 34.55 27.68 79.32 0.93 25.29 0.38 197.80 78.62 8.66 58.22 27.44 193.00 13.99 67.51 15.79 4.16 17.41 2.96 16.36 4.23 10.89 1.71 10.46 1.70

S1 142.07 24.50 912.57 180.17 84.28 39.07 21.95 24.83 37.96 229.43 1.04 175.38 0.84 28.25 42.10 8.11 68.19 25.94 205.96 14.16 78.90 18.60 4.50 18.38 3.19 17.68 4.17 11.66 1.60 11.08 1.71

S2 141.62 24.05 912.12 179.72 83.83 38.62 21.50 27.38 37.51 188.19 1.80 134.21 1.20 26.80 41.62 6.66 68.26 26.01 206.00 14.23 78.97 18.67 4.57 18.45 3.26 17.75 4.24 11.73 1.67 11.15 1.78

5.0

Samples/PAAS

Formation Member Facies Sample No. Ni Co V Cr Cu Zn Mo Pb As Zr Hf Nb Ta Th U Sc Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

2.5

0.0 La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Reduced facies (Dabdab Formation)

Oxidized facies (Dabdab Formation)

Reduced facies (Tarut Formation)

Oxidized facies (Tarut Formation)

Reduced facies (Ashkidah Formation)

Oxidized facies (Ashkidah Formation)

Fig. 5: PAAS-normalized REE diagram for the oolitic ironstones

Page 42

Oxidized S3 142.40 24.83 912.90 180.50 84.61 39.40 22.28 29.16 38.29 216.12 2.61 160.93 1.27 25.58 42.40 5.44 68.12 25.87 205.89 14.09 78.83 18.53 4.43 18.31 3.12 17.61 4.10 11.59 1.53 11.00 1.64

S4 196.97 195.40 592.47 305.07 4.18 93.97 1.85 34.73 17.86 131.98 1.47 76.84 0.77 306.15 76.97 6.00 76.56 32.73 211.55 16.33 78.05 18.52 4.47 19.75 3.28 18.83 4.23 12.51 1.68 11.54 1.80

S5 197.20 195.63 592.70 300.30 4.41 94.20 2.08 33.96 18.09 98.90 0.39 43.91 0.05 306.38 77.20 6.24 76.49 32.66 211.48 16.26 77.98 18.45 4.40 19.68 3.21 18.76 4.16 12.44 1.61 11.47 1.73

S6 197.31 195.74 592.81 307.41 4.52 94.31 2.19 35.07 18.20 117.86 1.35 63.03 0.70 306.49 77.31 6.35 76.63 32.80 211.62 16.40 78.12 18.59 4.54 19.82 3.35 18.90 4.30 12.58 1.75 11.61 1.87


Fig. 6: Ternary plots of SiO2–FeO+MgO–Fe2O3 for oolitic ironstone samples (fields after James, 1992) 2) Minette Type: This type is the most common and

the relatively high content of P2O5 (0.22-1.91), and low

widespread ironstones. The principal minerals are siderite and

TiO2/Al2O3 (0.04-0.13), may indicate a continental source for

chamosite. The Fe2O3 content is around 30%, while CaO runs

the phosphorous. Regarding the source of Fe in the studied

5-20% and SiO2 is usually above 20%. The Minette type is

oolitic ironstones, the authors also believe that Fe has been

common in the Mesozoic and Cenozoic of Europe, northern

leached from underlying sediments. This assumption is based

Africa and southern United States.

on the weak correlations between Fe2O3 and both P2O5 and Zn (r = -0.01 and -0.25, respectively). The oolitic ironstones clearly

The above statements suggest that the studied oolitic

have elevated SiO2, and Al2O3 but lesser MnO, and also lower

ironstones fall under the Clinton type.

amounts of CaO and MgO. The negative correlation of Fe2O3 with both SiO2 and Al2O3 (r = -0.9 and -0.8, respectively)

Origin

reflects the decreased deposition of detrital quartz grains and In general, there are different hypotheses regarding

fine grained detrital clay minerals during Fe-deposition, which

the source of the iron in oolitic ironstones. The Fe-enrichment

attest that iron minerals are primarily chemically precipitated.

can occur from supergene sedimentary processes (Macquaker

Si and Al data from the ironstone samples suggest a

et al., 1996) or the Fe-enrichment is hypogene including

hydrogenous origin based on their plotting in the hydrogenous

hydrothermal and/or volcanic sources (Garnit and Bouhlel,

field of the Si−Al discrimination diagram (Fig. 7).

2016).

Fe-Mn

hydrothermal,

oxyhydroxide hydrogenous,

precipitates, diagenetic

may or

be

of

mixed-type

The source of the ferromagnesian elements (Cr, Ni,

(diagenetic- hydrogenetic) origins, this terminology is based on

Co, Sc, and V) is likely to be from basic rocks; these elements

the type of aqueous fluid from which the Fe-Mn oxyhydroxides

can be supplied either from the weathering of the basic rocks

precipitate (Bau et al., 2014). High P2O5 content are recorded

present outside the basin or by within-basin volcanism (Khan

in several oolitic ironstone deposits that ranges from 0.2 to

and Naqvi, 1996). The affinity of Al, V, Cr, Mn, Mo and U for

0.8% but may sometimes exceed 1.5% (Chai et al., 2011). In

Fe-oxyhydroxides is well documented and manifests in

agreement with Baioumy et al., (2017) the authors believe that

different ways such as isomorphic substitutions or surface

Page 43


40

Si %

30

Hydrothermal

20

Hydrogeneous

10

0 0

2

4

6

8

10

Al % Reduced facies (Dabdab Formation)

Oxidized facies (Dabdab Formation)

Reduced facies (Tarut Formation)

Oxidized facies (Tarut Formation)

Reduced facies (Ashkidah Formation)

Oxidized facies (Ashkidah Formation)

Fig. 7: Bivariate plots between Al vs. Si in the ironstone samples (fields after Choi and Hariya, 1992) adsorption (Cornell and Schwertmann, 2003). Hydrothermal

evidence of volcanic activity in the basin when these Fe-oolites

Fe-Mn deposits show higher contents of Zn, Pb, Mo, V and As

formed. Further, the absence of volcanic material in the studied

and are depleted in Co, Ni and Cu relative to hydrogenous

samples also suggests a non-volcanic origin for the Fe-oolites.

deposits (Boyd and Scott, 1999). The discrimination diagram

La/Ce ratio in the studied samples is 0.14, in average, which is

based on Ni+Co vs. As+Cu+Mo+Pb+V+Zn also indicates that

very close to hydrogenous Mn−Fe crusts (0.25, Nath et al.,

the studied ironstones display hydrogenetic type mineralization

1997).

(Fig. 8). The hydrogenous and hydrothermal deposits can be also distinguished by using Co/Ni and Co/Zn ratios (Toth,

Depositional Environment and Paleooxygenation

1980). A ratio of Co/Ni < 1 and Co/Ni > 1 indicates a sedimentary

origin

and

a

deep

marine

The depositional environment of oolites has long been

environment,

a subject of speculation and discussion. Several depositional

respectively (Oksuz, 2011). A ratio of Co/Zn of 0.15 is

environments have been proposed for oolitic ironstone: shallow

indicative of a hydrothermal type deposit and a ratio of 2.5

marine (Garnit and Bouhlel, 2016), offshore transition marine

indicates a hydrogenous type deposit (Toth, 1980). In the

(Burkhalter, 1995); restricted lagoonal marine (Bayer, 1989); or

ironstone samples, Co/Ni and Co/Zn ratios range from 0.17 to

coastal and deltaic setting environments (Collin et al., 2005).

0.98 and 0.62 to 2.12, respectively. Although Co/Zn ratios point

They are usually encountered in simply folded shallow shelf

to a hydrogenous source for Fe mineralization, Co/Ni ratios of

areas, and most typically are close to the transition from

the samples indicate that sedimentary environments played an

nonmarine to marine environments and always hosted by

important role during the formation of the Fe deposits.

clastic sediments at the top of coarsening and shoaling-upward cycles (Maynard and Van Houten, 1992).

A positive Eu anomaly and low ∑REE (2), Ni/Co (>5) and U/Th (>1.25) ratios. The

The present paper work describes the mineral and

discrimination diagrams based on SiO2 vs. Al2O3+N2O3+ K2O

chemical compositions of the oolitic ironstones at Wadi As

and CIA vs. K2O/Na2O indicated semi-humid to semi-arid

Shati, SW Libya. The three formations; Dabdab, Tarut and

conditions. Direct dating results of molybdenite from the

Ashkidah, represent the most important iron ore bearing cycles

ironstone deposits using Re-Os isotope systematics show that

of sedimentation in the Wadi As Shati area. The studied

the ages of Dabdab, Tarut and Ashkidah formations are

ironstones are divided into two facies, reduced and oxidized.

Fransnian (Late Devonian), Famennian (Late Devonian) and

The detected iron minerals are goethite, siderite, hematite,

Tournaisian (Early Carboniferous), respectively.

limonite,

magnetite,

chamosite

and pyrite.

The

oolitic

ironstones fall under the Clinton type. Regarding the source of

Acknowledgements

Fe in the ironstones, the authors believe that Fe has been

The authors thank the Thin Section Lab, Toul, France

leached from underlying sediments. This assumption is based

for thin section preparation, the ACME analytical laboratories

on the weak correlations between Fe2O3 and both P2O5 and

of Vancouver, Canada for atomic absorption spectroscopy, LOI

Zn. The discrimination diagrams based on Al vs. Si, Ni+Co vs.

and ICP-MS analyses and the Institut de Physique du Globe

As+Cu+Mo+Pb+V+Zn, ∆Ce vs. Nd and ∆Ce vs. (Y/Ho)N

de Paris, France for N-TIMS analysis.

indicate that the studied ironstones display hydrogenetic type mineralization. ΔCe values are weakly correlated with Pb, in

References

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Akinlua, A., Olise, F.S., Akomolafe, A.O. and McCrindle, R.I.

oxidized facies display low values of V/Cr (